1. Field of the Invention
This invention relates to an apparatus and process for thermal treating using convection thermal transfer for elevated processing temperatures and may provide controlled chemical atmosphere for chemical treatment, such as carburizing. An elongated treatment chamber has a decreasing cross sectional area along its length, thereby providing introduction of treatment fluid at different locations to achieve desired uniform thermal and chemical treatment of articles along the length of a treatment chamber. A recirculation plenum, for passage of treatment fluid to and from a treatment chamber, contains a thermal control source and blower and may contain a chemical control source to provide predetermined programmable temperatures and chemical environments, such as oxidizing, reducing, neutral or other desired chemical environment, to treatment fluid for passage to a treatment chamber to effect desired thermal treatment and chemical surface reactions to articles passed through the treatment chamber. The uniform flow of treatment fluid obtained by this invention provides even temperatures and uniform chemical treatment of articles along the length of a treatment chamber reducing, or eliminating, uneven chemical treatment and thermal distortion of articles being treated. This invention is particularly applicable to thermal treatment and combined thermal and chemical treatment of metal articles, such as carburizing.
2. Description of Related Art
Heating metal articles, for example, for heat treating with heat sources and fans in a treatment chamber is well known. U.S. Pat. No. 1,949,716 teaches heat treating metal objects in a furnace having electric heating elements along side walls of a treatment chamber and a plurality of fans above and/or below the objects being treated. U.S. Pat. No. 4,767,320 teaches a continuous heat treating furnace compartmented into preheating, treating and cooling zones with the preheating and cooling zones divided into a plurality of compartments with controlled recirculation of gases from specific cooling zones to specific preheating zones for heat recuperation. All zones of the furnace have convection fans and the treating zone has a plurality of direct fired burners.
Heating metal articles for heat treatment wherein fans and heat sources are separated from the treatment chamber is exemplified by the teachings of the following patents. U.S. Pat. No. 5,127,827 teaches a horizontal cylindrical furnace having one closed end and an access door at the opposite end. Spaced inwardly from the closed end is a fan plate separating a heat treating chamber from a fan chamber, the fan plate having a central opening for recirculation of gases from the treating chamber to the fan chamber and a peripheral annular opening for circulation of gases from the fan chamber along the periphery of the treating chamber to the opposite end and return passage across parts being treated in the central portion of the treating chamber to the central opening in the fan plate. In one embodiment, heating elements extend in the periphery the length of the fan chamber and the treating chamber, which additionally is provided with heat sinks, and in another embodiment a peripheral incineration burner with an incineration combustion zone extends around the periphery of the fan chamber and a peripheral gas fired heating burner is fired into the fan chamber, both burners providing heat to gases in the fan chamber for circulation to the treating chamber. The atmosphere in the treating chamber may be controlled by the combustion conditions of the heating burner. U.S. Pat. No. 4,751,886 teaches a control system for ramping temperature in the main chamber of a batch pyrolysis furnace and duration of subsequent soak by a single thermocouple in the throat of the furnace controlling water spray to the main chamber. The furnace is heated by hot combustion gases from a separated main burner in combination with an afterburner. U.S. Pat. No. 4,664,359 teaches a vertical furnace wherein alloy ingots are conveyed through a plurality of partitioned vertical heating shafts which are heated by circulation of gases driven by a fan in a separate vertical plenum in heat exchange relation with an electric heater or flames from combustion burners. U.S. Pat. No. 4,219,324 teaches heat treating metals in a horizontal furnace in which radiant tube firing at a low pressure is the source of heat as well as the source of desired atmosphere composition for the heat treatment. The treatment chamber is in indirect thermal exchange relation with radiant tube heaters from which the combustion product gas is externally treated and passed to the treatment chamber as a treatment atmosphere. One disadvantage of this system is that the combustion product gas is cooled in the external treatment system.
Heat treatment systems having a plurality of temperature and/or controlled atmosphere zones are known for a wide variety of applications. Heat treatment of metal parts is taught by U.S. Pat. No. 4,932,864 wherein a roller hearth heat treating furnace has three separated independently temperature controlled treating chambers, the first chamber having radiant tubes and fans, the second having radiant tubes, a cooler and fans, and the third having electro tubes, cooling tubes, and fans. U.S. Pat. No. 4,857,689 teaches heat treating of semiconductor wafers in a tubular furnace having indirect heating at a closed end and injection of cooling gas at the opposite end wherein very rapid programmable heating and cooling is obtained by movement of the semiconductor wafers along the axis of the furnace in response to temperature sensors. U.S. Pat. No. 4,218,214 teaches a furnace paddle for holding semiconductor wafers for movement through a cylindrically-shaped processing furnace. U.S. Pat. No. 4,744,750 teaches a tunnel kiln having a heat-up zone, firing zone, and cooling zone wherein kiln cars having a solid bottom extending close to the side walls forms an undercar cooling channel in the cooling zone and downstream end of the firing zone. The cooling zone is cooled by fans recycling air over heat exchangers providing convection cooling of the material being treated. U.S. Pat. No. 4,310,300 teaches a porcelain enameling furnace system having a drying zone, preheat zone, heating zone, and cooling zone wherein the heating zone is heated by a serpentine tube radiant heater with combustion gases passing from the radiant heater in indirect heat exchange in the preheat zone and then passing in direct heat exchange with the articles being treated in the drying zone. U.S. Pat. No. 3,802,832 teaches a heating chamber formed from a number of sub-chambers which are releasably connected to modify the capacity of the heating chamber for heat treatment of food. The chamber is heated by internal heaters with air circulated by fans.
Hot pressing of metal powder is taught by U.S. Pat. No. 4,689,008 wherein a system for rapid heating of metal powder for consolidation has a plurality of zones of controlled temperature and atmosphere wherein the powder is preheated, heated by hot cavity walls, packed, and consolidated by pressing. U.S. Pat. No. 3,971,875 teaches vacuum hot pressing of materials wherein an external press acts upon rigid walls of the furnace chamber with movable sidewalls to provide application of the press force upon the material in the chamber.
U.S. Pat. No. 4,472,887 teaches a series of separated chambers individually heat and moisture controlled for dehydrating produce, each chamber having a separate plenum in which a fan recirculates air past the flame of a gas combustion burner for heating and reintroduction to one end of the treatment chamber.
Roller conveyors are known for transporting articles in heat treating furnaces. U.S. Pat. No. 5,002,009 teaches a tunnel furnace having a roller conveyor with independently controlled drive systems in separate chambers of the furnace for conveying containers through the furnace in adjacent fashion. U.S. Pat. No. 4,807,853 teaches a continuous furnace for gas carburizing and hardening having a plurality of groups of roller conveyor drive systems for conveying articles at different speeds through different portions of the furnace. U.S. Pat. No. 2,978,237 teaches a heat treating furnace having continuously operating conveying rollers for transport of articles and means for lifting the articles from the conveying rollers for desired periods of treatment. U.S. Pat. No. 4,802,844 teaches a retractable roller hearth to take the load off rollers during most of the heat cycle in a batch heat treating furnace having a radiant tube within the treating zone to prevent commingling of combustion gases and furnace atmosphere.
Pusher mechanisms are known to be used for transport of trays of parts in heat treating furnaces. U.S. Pat. No. 5,143,558 teaches an integrated continuous/batch furnace system wherein trays of parts are individually loaded and pushed adjacently in the continuous furnace system and are connected together with clips and delivered to the batch furnace system. The continuous furnace system uses a rotary carburizing furnace. U.S. Pat. No. 4,582,301 teaches heat recovery in a heat treatment furnace system wherein baskets of articles are pushed by pistons through the furnace system. Heat transfer gas is constrained to flow vertically through the baskets and the ends of the baskets fit gastight in abutment with the end of adjacent baskets. U.S. Pat. Nos. 3,662,996 and 3,598,381 teach carburizing furnace systems wherein trays of materials to be treated are pushed on rails through the system by a number of motor driven pushers.
U.S. Pat. No. 2,842,352 teaches transport of a tray of work in a batch heat treating furnace by a reciprocating rod having hook(s) for engagement of and disengagement from the tray by rotation of the rod 90.degree., the reciprocating rod pulling the tray into position in the furnace on rails. A plurality of hooks may be used for engagement at opposite ends of the tray and for shortening the stroke of the reciprocating rod which is driven by a mechanism exterior to the furnace.
U.S. Pat. No. 4,245,818 teaches transfer devices providing independent speeds through pretreatment and heat treatment furnaces. The pretreatment furnace uses rapid hot gas impingement heating and the holding furnace uses forced hot gas circulation. The articles are rolled across saw-toothed shaped depressions on the transfer devices.
Transfer of materials directly from transport through a heat treatment furnace to a lower quench bath is taught by several U.S. Pat. Nos. 2,747,855 teaches transport of articles from a heat treatment furnace by an elevator to a quenching zone while being maintained in a controlled atmosphere common to the treatment furnace and the quenching zone; 3,191,919 teaches a chain driven transfer mechanism moving articles through a heat treatment furnace and onto an elevator for lowering into a quench tank; and 3,718,324 teaches a work cart which moves through a vacuum heat treatment furnace to an elevator for lowering into a quench chamber, the heating zone, transfer zone, and quench zone being open to each other. U.S. Pat. No. 2,367,732 teaches a work holder for vertical reciprocation in a liquid treatment tank.
It is known to separate a heat treating zone from a quenching zone as taught by the following U.S. Pat. Nos. 3,484,085 teaches an elevator to transport material from a heat treatment furnace to a quench chamber directly below the heat treatment furnace and separated by a retractable door, with the material support means used in the heat treatment furnace not being transferred to the quench chamber; 3,219,330 teaches a reciprocating pusher rod insertable through the furnace door moving material through and from a sealed heat treating furnace through discharge doors to an elevator for lowering into a quench bath; and 2,777,683 teaches introducing material into a vestibule and pushing the material from the vestibule into an electrically heated heat treatment furnace and use of chain driven roller means for conveying material through a controlled atmosphere vestibule chamber to an elevator for lowering into a cooling treatment tank with the openings in the bottom of the vestibule for the elevator being sealed from outside environment by casings extending into the cooling liquid.
U.S. Pat. No. 3,950,192 teaches carburizing of a continuous strip in a heat treating furnace providing high carbon availability followed by homogenizing and quenching by impinging gas jets. Carburizing systems having multiple zones is taught by U.S. Pat. No. 4,622,006 for heat treating metallic articles, especially two-stage carburization, in a furnace having a plurality of treatment chambers in which the articles are irregularly conveyed in at least one of the chambers resulting in varying retention times in the chambers, such as joined rotary-cycle furnaces having selectively rotatable hearths. The cycle for carburization includes a preheating chamber, carburization chamber, diffusion chamber, cooling chamber, and quenching chamber. U.S. Pat. No. 5,164,145 teaches a rotary carburizing furnace having an annular fluid seal in the annular slot between the hearth and the sidewall. Internal radiant tubes provide heat which is distributed by fans and atmospheric inlet ports to the furnace chamber with gas ports delivering purge gas to annular slot. The carburizing system includes a separated preheat furnace, rotary carburizing furnace, rotary diffusion furnace, and rotary equalizing furnace. U.S. Pat. No. 4,627,814 teaches a carburizing continuous heat treating furnace for metal parts which is separated into a charging chamber, heating chamber, treating chamber, and cooling chamber, each chamber having thermal transfer means and fans within the chamber. The charging chamber is fed air and combustible gas to create desired oxidizing atmosphere for deoiling and the other chambers are provided with a gas generator for endothermic gas to create neutral and reducing atmospheres.